1. Field of the Invention
This invention relates to an annular tube-like driver made of a shape-memory alloy for insertion into a tube insert which in turn is inserted into a tube.
2. Discussion of the Prior Art
In tube-type heat exchangers a first fluid flows through the heat exchanger tubes while a second fluid is in heat-exchange relationship therewith. If one of the tubes becomes defective a leak can occur which is obviously an unsatisfactory situation. In this situation it is sometimes necessary to plug the defective tube so that fluid does not flow therethrough, thereby preventing leakage and correcting the defective condition. In addition, it is sometimes necessary to provide a liner to the inside of a tube.
If such a defect occurs in the steam generator tubes of a nuclear reactor power plant, a very serious situation can result. When such a defect occurs in these tubes it not only allows coolant in the tubes to mingle with fluid outside the tubes, but also creates a radioactive contamination problem.
It is clearly quite important not to allow fluid to leak from the tubes and contaminate the surrounding fluid since the fluid in the tubes is generally radioactive. Plugging or sleeving the offending tube solves the problem and prevents contamination.
Mechanical plugging devices are commonly used to plug heat exchange devices, such as that shown in U.S. Pat. No. 4,390,042. Mechanical devices, however, inherently depend upon mechanical installation and the skill of a particular installer. Therefore, they are not always entirely effecttive. Also, mechanical devices can be and often are difficult to install and remove, both of which represent serious problems.
In addition, providing a liner to the inside of a tube is a difficult procedure, especially where the tube is located in an operating situation.
U.S. Pat. No. 3,900,939 utilizes a tube plug fabricated from a metallic alloy having a mechanical memory characteristic, i.e., a shape-memory alloy, whereby it can be physically deformed into a reduced diameter for insertion into the defective tube and thereafter induced to assume its original, larger diameter to secure the plug within the tube. However, this patent relies solely on the shape-memory alloy to plug the tube and it may not be entirely effective for this purpose and presents severe removability problems.
Materials, both organic and metallic, capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to have shape memory for the reason that, upon the application of heat alone, it can be caused to revert from its heat-unstable configuration to its original, heat-stable configuration, i.e., it "remembers" its original shape.
Among metallic alloys the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change of temperature. Also, the alloy is considerably stronger in its austenitic state than in its martensitic state. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example, a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as the M.sub.s temperature. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as the A.sub.s temperature, the deformed object will begin to return to its original configuration.
Shape-memory alloys have found many uses in recent years. For example, pipe couplings such as are described in U.S. Pat. Nos. 4,035,007, 4,198,081 and 4 309,807 and British Patent Nos. 1,553,427, 1,554,431, 1,554,432, 1,554,433 and 1,488,393, electrical connectors such as those described in U.S. Pat. No. 3,740,839, switches such as those described in U.S. Pat. No. 4,205,293 and fastening devices such as are described in British Patent No. 1,548,964.
It would thus be desirable to develop a method for providing a liner to the inside of the tube, and also to develop a method for plugging a tube, all in a convenient, economic and efficient manner, amenable to remote installation with robotics. It would also be desirable to develop an improved removable structure for achieving the foregoing.